Black hole X-ray binaries are powered by an accretion disc. The X-rays are emitted from the inner regions of the disc close to the black hole where the characteristic time-scales are expected to be of the order of less than seconds. Thus, it was rather surprising that the X-ray emission from these systems vary over a wide range of time-scales. It is now believed that this wide range occurs because viscous fluctuations with long characteristic time-scales occur in the outer parts of the disc and subsequently these variations propagate to the inner parts [1]}. Such a model qualitatively explains the power-law form of the power spectra of the X-ray emission. Moreover, these long time-scale fluctuations arising from large radii on propagating inwards gets superimposed on the smaller time-scale fluctuations arising there. Hence, this explains the linear positive relation between the \(r.m.s\)  variability and the X-ray flux, that are observed in many X-ray binaries [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. Thus, the model provides a natural connection between the power spectra and the rms-flux relation which has been studied by several authors [9]}, [8]}. Moreover, the same process should be acting on accreting systems such as Active Galactic Nuclei and hence in general can be used to obtain the size scale of the system [11]}, [12]}. By modeling the observed fluctuations, it may also be possible to obtain direct constraints on the hydrodynamic structure of the accretion disc [13]}.
